1. Field of the Invention
The present invention relates to a fluid-filled insulating bushing used in a suspension arm and the like of an automobile.
2. Description of Background Art
Such a fluid-filled insulating bushing is well known, for example, as disclosed in West German Patent No. 2841505. I this bushing an inner cylinder, an outer cylinder and an elastic block are provided between the cylinders. Fluid chambers are divided into plural numbers in a circumferential direction by elastic walls and communicate with each other through an orifice passage being formed in an elastic block. In the present invention, the circumferential direction is defined as a concentric direction of the circumferential direction of the inner and outer cylinders, an axial direction is defined as a direction parallel to the axial direction of the inner and outer cylinders and a radial direction is defined as a direction which coincides with the radial direction of the inner cylinder.
Now, the elastic wall is formed thinner to a certain extent so that it is susceptible to elastic deformation, and acts to absolve the pressure rise following a volumetric change of the fluid chambers, thereby vibration of a small amplitude is coped with a mild spring characteristic or a low-action spring, and a high attenuation is exhibited against vibration of a large amplitude. However, since a large fluid pressure is repetitively exerted on the elastic wall, the base portion of the elastic wall is liable to fatigue, so that a spring constant at this portion can not be reduced unlimitedly and low-action spring characteristics are naturally restricted definitely. Thus, it is strongly desired to improve the low-action spring and high attenuation characteristics and to improve durability by relieving the concentration of stress against the elastic wall. The present invention is directed to satisfying such requirements.